RF amplifiers, mixers, and baseband converters are well known in the art of high frequency signal processing. One of the characteristics of an RF receiver is gain control, and one of the limitations of the output of an RF receiver is saturation, whereby the signal level is amplified beyond the linear region of operation of the amplifier. Typically, in an RF receiver, each successive stage generates signal gain, and in a digital signal processing system which accepts as input the A/D outputs of the RF receiver. For best noise performance, each stage has a gain characteristic which exceeds its noise contribution, so that in well-designed systems, it is usually the last stage of the amplifier chain which saturates from excessive gain for a given input signal level. For a digital system, this last stage is the A/D converter, and it is possible to measure the signal level generated by the A/D converter and determine whether to increase or decrease the gain of the system. For a wireless communications system operating under the IEEE 802.11 series of standards, it is further desired to make gain adjustments during the initial stage of the packet known as the preamble.
FIG. 1 shows an IEEE 802.11 wireless Ethernet packet 10. The IEEE standards have provided an initial set of specifications for 802.11, and subsequent revisions to the specification have added, and continue to add additional standards, which are reflected in a suffix letter. The group of standards having the 802 prefix are known collectively as Ethernet standards. Currently approved IEEE standards for wireless communications are IEEE 802.11, IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, and others are in development. In the present invention, we will refer to the entire body of current and future IEEE 802.11 standards simply as 802.11. The wireless packet 10 comprises a preamble 12 which has a preamble time T1 16, and is followed by the data frame 14 which occupies a data time T2 18. The communications system receiving this packet has the duration of the preamble T1 16 to accomplish synchronization and set the gain control. For 802.11b T1=15 us, and for 802.11a T1=6 us. Prior art Automatic Gain Control (AGC) systems typically accommodate slow changes in AGC level over long intervals of time. In 802.11 wireless communications, the distinction is that the information arrives in discrete packets from different transmitter sources, and each packet may require its own AGC level.
FIG. 2a shows a prior art RF receiver, as might be used in an 802.11 wireless system. This type of system is known as a dual conversion receiver, and comprises an antenna 22 for receiving incoming signals, an RF receiver 20 for processing and baseband converting these signals into a quadrature pair of signals 21 and 23, and analog to digital (A/D) converters 24 and 26 for converting these signals into a pair of quadrature digital outputs 25 and 27, each with a data width Ndata. There are several signal processing methods for realizing an RF receiver, and prior art FIG. 2a shows a dual conversion receiver. Incoming signals are amplified by preamp 28, which has a gain control input for receiving an analog signal from a D/A converter 29, which produces this gain control signal from a digital preamplifier gain port 50. This gain control input 50 causes a varying amount of gain to be generated by preamplifier 28, which delivers the amplified signal to a mixer 30, which frequency converts the RF signal to an intermediate frequency (IF) with the use of a first local oscillator 32. An IF filter 34 removes the unwanted image frequencies, and the IF amplifier 36 amplifies the IF signals and provides them to a pair of mixers 38 and 42. Quadrature local oscillators 40 and 44 produce a pair of baseband detection signals which are phase coherent single frequency signals separated in phase by 90 degrees. These quadrature signals are provided to the mixers 38 and 42, and low pass filters 46 and 48 remove all but the quadrature baseband signals 21 and 23, which is suitable for conversion with A/D converters 24 and 26. In a typical RF system, preamplifier 28 has a small amount of gain control, and IF amplifier 36 has a larger amount of gain control. The characteristic of gain control is often exponential with applied voltage, producing a gain/control characteristic of db/volt. An RF receiver where the gain control has the characteristic of increasing gain with increased control is known as a “positive gain control amplifier”, and an RF receiver where the gain decreases with increased control is known as a “negative gain control amplifier”.
FIG. 2b shows a prior art direct-conversion RF receiver 60. Signals from an antenna 62 are delivered to the RF receiver 60, and are applied to variable gain preamplifier 64, as before. The amplified signals are applied directly to a quadrature detector comprising a pair of mixers 68 and 72, which are driven by quadrature local oscillators 70 and 74. Matched low pass filters 76 and 78 produce signals for the A/D converters 80 and 82, which simultaneously sample the signals to produce I (in-phase) 81 and Q (quadrature) 83 digital signals, each with a word size of Ndata. Gain control port 66 operates as before, where a digital signal is sent through a D/A converter to produce a control signal for preamplifier 64.
FIG. 3 shows the nature of gain control for an exemplar system. An arbitrary signal level is applied to the antenna of the systems of either FIG. 2a or 2b, and produces an output at the A/D converter. For optimal performance, sufficient gain control should be applied to the RF receiver to move the signal to the range noted as optimal RMS (Root Mean Square) level 91, which corresponds to an “optimum gain control input range” 90. This is accomplished by applying the suitable level of gain control to achieve the optimum RMS input level 91, which depends on whether the amplifier has a positive gain control characteristic 92, or a negative gain control characteristic 94, as well as the sensitivity and saturation characteristic of the gain curve. A digital gain control input Vagc 96 is applied which exponentially changes the gain according to 92 or 94, and the gain control range 90 which causes the A/D to receive optimum RMS voltage levels is the desired gain control range 90. This level must be reached during time T1 of FIG. 1, while still allowing enough time for system synchronization to occur on the preamble 12.
It is desired to have an AGC apparatus and method for an RF receiver having a gain control and digitized outputs which acts upon the digitized outputs and provides the gain control signal to quickly bring the gain of the RF receiver to a level sufficient to allow subsequent signal processing. It is further desired for the AGC apparatus and method to settle to a usable level of gain during the time of reception of the preamble of the wireless packet.